<!DOCTYPE html> 
<html lang="en"> 
<head> 
<meta charset="UTF-8"> 
<meta name="viewport" content="width=device-width, initial-scale=1.0"> 
<title>let 命令</title> 
<script> 
{ 
var x = 20; 
 let y = 10; 
 } 
 console.log(x); // 20 
 console.log(y); // ReferenceError: y is not defined 
 </script> 
 </head> 
 <body></body> 
 </html> 
